Particle blast cleaning apparatuses are well known in the an and it is known to use various types of blast media, such as ordinary sand, tiny glass beads, walnut shells, peanut husks, etc. One difficulty with these types of media is that the spent media can be difficult and time consuming to clean up.
At least partially due to the difficulty of gathering and disposing of the spent media, the art has turned to the use of sublimable particles as a blast media. Specifically, it has become known that carbon dioxide (CO.sub.2)ice(i.e., "dry ice") can be used as a blast media. Dry ice, being in solid form, acts as a good abrasive media. After being spent during the cleaning operation, the dry ice simply evaporates and leaves nothing to be cleaned up other than the residue which has been abraded from the surface of the object being cleaned. This substantially reduces the task of cleaning up after the blast cleaning operation. Unfortunately, dry ice exists at extremely low temperatures, such as below more than -100.degree. F. This proves to present certain practical difficulties when the extremely cold blast media is used in the blast equipment. The dry ice rapidly absorbs heat from the surrounding equipment, thereby substantially lowering the temperature of the equipment. This can result in the formation of ice condensation on the outside or inside of the equipment and can, through thermal contraction, substantially change the dimensions of critical components of a device.
Various efforts have been made in the art to provide a workable sublimable media blast cleaning apparatus. One of the difficulties encountered in the art is reliably introducing or entraining dry ice particles in a fast flowing stream of propellant gas. In general, two arrangements are typical in the art. In one arrangement, a rotary pellet transport arrangement introduces dry ice pellets into a high pressure gas stream through the use of a discharge nozzle. Such an arrangement is generally disclosed in U.S. Pat. No. 4,617,064 of Moore. In a typical rotary arrangement, cavities formed in a rotating member are filled with dry ice pellets at a receiving station and transported to the discharge station where they are entrained in the propellant gas stream. Another such rotary arrangement is shown in U.S. Pat. No. 4,947,592 of Lloyd et al. Rotary-feed devices are particularly prone to "freeze-up", a phenomenon common in applications involving low temperature dry-ice media, wherein thermal contraction and frozen condensate prevent the device's moving parts from operating as intended.
In another type of arrangement known in the art, a plurality of feeder bars move back and forth in linear fashion to collect dry ice pellets and transport them from an inlet to an outlet where they are entrained in the high speed gas flow. Such an arrangement is shown in U.S. Pat. No. 4,741,181 of Moore, et al. The '181 Moore, et al. arrangement generally suffers from having a large number of moving parts, thereby being rather complex. Because of the low temperatures attendant upon the use of dry ice, mechanical complexity should be avoided in order to maintain good reliability.
In general, among the problems experienced in the art are: (1) maintaining a relatively uniform and reliable flow of dry ice particles; (2) evenly mixing the dry ice particles with the flow of compressed gas; and (3) providing a device that operates reliably without "freeze up". It is to the provision of such an apparatus that overcomes these problems that the present invention is primarily directed.